The commercial radio spectrum is populated by government-licensed commercial radio stations operating at designated center frequencies. For example, the FM radio spectrum is segregated into 102 channels in the range of 87.5 MHz to 107.9 MHz, the channels being evenly spaced at 0.2 MHz intervals. To prevent interference, each radio station is allocated a portion of frequency space or bandwidth around its designated center frequency. The FCC has specified a "mask" identifying the amount of transmitting power a station may produce at each wavelength; most of the transmitting power must be near to the center frequency; progressively lower power is permitted at wavelengths away from the center frequency.
There is increasing interest in using bandwidth available to FM stations to transmit additional information, such as digital broadcast data; this is known as an In Band On Channel (IBOC) transmission. Typically, additional information would be added to the FM transmission by modulating the amplitude of the FM carrier signal, or by summing a non-interfering signal within the allowed mask, resulting in additional transmission power in sidebands surrounding the existing 0.2 MHz band of FM program material.
One known technique for transmitting digital data over a radio channel is known as Code Division Multiple Access (CDMA) transmission. This technique is used to transmit subsignals of digital information from multiple transmitters over a single channel for reception by multiple receivers at different and possibly mobile locations.
To send each positive or negative digital signal of the subsignal, a CDMA transmitter respectively sends a code sequence or an inverted version of the code sequence. Typically, the code sequence is itself a sequence of digital signals each having a positive or negative value; to send the code sequence, a first analog output level is transmitted for every positive digital signal in the code sequence and a second analog output level is transmitted for every negative digital signal in the code sequence. To send an inverted version of the code sequence, the first output level is transmitted for every negative digital signal in the code sequence, and the second output level is transmitted for every positive digital signal in the code sequence.
A receiver selects a desired CDMA transmission by correlating the signals it receives on the CDMA channel with an internally-generated version of the desired code sequence used by the desired transmitter. To correlate the signals, the receiver first multiplies the analog signals received on the CDMA channel by the desired code sequence, i.e., the received signal is inverted during negative digital signals of the desired code sequence and not inverted during positive digital signals of the desired code sequence, producing a product signal. To complete the correlation, the receiver maintains a summation of the product signal over the sequence period. When the receiver reaches the end of the desired code sequence, the receiver uses the accumulated sum to determine the value of the transmitted digit, returns to the beginning of the desired code sequence, and clears the sum to zero. If the internally-generated desired code sequence is in phase with the code sequences produced by the transmitter, the sum will be a large positive or negative number, corresponding to a positive or negative digital signal of the subsignal from the desired transmitter.
For such a scheme to work properly, the code sequences used by the CDMA transmitters must satisfy two requirements: first, the code sequences must be approximately orthogonal to each other. Approximately orthogonal digital signal sequences have approximately the same number of unequal digital signals as equal digital signals (comparing corresponding digital signals from each sequence) when compared in any phase to each other. Two sequences having this property relative to each other will be referred to as having near-zero cross-correlation. This requirement arises from the fact that the code sequences are transmitted by multiple transmitters at different locations, and therefore receivers located in different geographical positions relative to the transmitters will receive sequences in substantially different phase relationships. Requiring approximately orthogonal sequences ensures that, when a receiver correlates the signals on the CDMA channel with a desired code sequence, any other code sequences on the CDMA channel, regardless of their relative phase, will have minimal impact on the correlation performed in the receiver.
The second requirement is that the CDMA code sequences must have near-zero auto-correlation; that is, each sequence, when compared to any out-of-phase version of itself (i.e., a version formed by repetitively barrel-shifting the last digital signal of the sequence to the beginning of the sequence), must have approximately the same number of equal digital signals as unequal digital signals. This requirement ensures that a receiver can synchronize its internally-generated code sequence to the code sequences being transmitted by the transmitter. If the internally-generated code sequence is out of phase with that being transmitted by the transmitter, when the internally-generated code sequence is correlated to the signal received on the CDMA channel, an approximately zero result will be obtained. However, if the internally-generated code sequence is in phase with the transmitter, the correlation will produce, as noted above, a relatively large positive or negative result. Thus, the receiver may synchronize to the transmitter by slowly varying the phase of the internally-generated code sequence, until large positive and negative results are obtained.
So-called "Gold" code sequences have near-zero out-of-phase autocorrelation and cross-correlation, and thus are suitable candidates for CDMA transmission. Gold code sequences can be derived from known mathematical formulas. However, neither Gold code sequences, nor any other known set of sequences used in CDMA transmission, have perfect orthogonality (i.e., when phased versions of two Gold code sequences are compared, there is never exactly the same number of equal and unequal bits). Rather, Gold code sequences and other sequences used in CDMA transmission are only approximately orthogonal. However, Gold code sequences are closer to the orthogonal ideal than other types of sequences, and therefore are often used in CDMA systems.
Another type of code sequence is the "maximal length" code sequence. The out-of-phase autocorrelation of a maximal length code sequence is typically much lower than a Gold code sequence, however, sets of maximal length sequences have greater cross-correlation than sets of Gold code sequences; thus CDMA systems typically use Gold code sequences rather than maximal length code sequences.
Maximal length code sequences, and other forms of code sequences, are described in Spread Spectrum Systems with Commercial Applications by Robert C. Dixon, John Wiley & Sons, Inc., New York, ISBN 0-471-59342-7, in particular in the chapter entitled "Coding for Communications and Ranging".
One difficulty with CDMA schemes is that they exhibit limited throughput; that is, the number of sequences is much larger than the number of digital signals, or bits, in the sequences. For example, it is believed that there are less than seven different 63-bit sequences having sufficient cross-correlation and autocorrelation characteristics to be used in a conventional CDMA environment (sets of four or more 63-bit sequences can be generated, but the sequences are not believed to demonstrate the necessary orthogonality and autocorrelation for a robust system). Therefore, using 63-bit sequences in conventional CDMA techniques, only 3 sequences can be simultaneously transmitted, limiting the rate at which digital signals of the subsignal which can be transmitted.